1. Field of the Invention
The invention lies in the field of recovery of aluminum from alunite.
2. Description of the Prior Art
In prior art processes for the recovery of aluminum from alunite ore, it is conventional to prepare a residue containing aluminum valves for a Bayer digestion step by dehydrating the ore and removing sulfur from it. In this procedure, it is necessary to grind the ore to about -20m for fluid bed reactors in which the particulate ore is roasted to remove water and some of the sulfur from the aluminum sulfate as sulfur dioxide. Potassium hydroxide is conventionally added to convert the remaining sulfate to soluble potassium sulfate in which form it is removed. The roasting converts the aluminum sulfate to aluminum oxide for the Bayer leach which converts it to soluble aluminates from which aluminum values are recovered as aluminum hydroxide.
Although the prior art processes as outlined above are commerically feasible, they are subject to improvements which will enhance their commerical possibilities.
One disadvantage is the fact that the fluid bed reactors which must be used for dehydration and desulfurization of the ore are very large and involve high capital costs. The ore must ground to -20m for use in the fluid bed reactors, and this involves large capital and operating costs in dry grinding. Large operating costs are involved in the potassium hydroxide addition which must be added in amounts of 20-50 pounds per ton or ore and addition in these amounts produces large amounts of by-product potassium sulfate that is difficult to dispose of profitably. Further, large capital costs are involved in operating the Bayer leach circuits required to achieve "sandy" alumina from the low A/C (aluminum to caustic) ratio liquors resulting from prior art procedures. Further, prior art procedures result in a large mud load for a Bayer plant and the sulfate contamination of the Bayer liquor resulting from ineffective removal of sulfate in the preparatory procedure contributes to large operating costs. Additionally, prior art procedures require a separate desilication section to remove the large amounts of silica carried over in the Bayer leach liquor before aluminum hydroxide can be precipitated from it.
Accordingly, it would be advantageous to have available a process for recovering aluminum from alunite in which in the preparation of the ore for the Bayer leach, fine grinding is not required, sulfate is effectively removed without the use of potassium hydroxide, the mud load on the Bayer plant is reduced, a Bayer leach liquor is produced having a high A/C content, and silicon dioxide is removed from the ore residue prior to the Bayer leach so that a separate desilication section is not necessary for removing silicon compounds from the Bayer leach liquor before aluminum hydroxide can be precipitated from it.